1. Field of the Invention
The present invention relates to a laminate, a substrate with wires, an organic EL display element, a connection terminal for the organic EL display element and a method for producing each.
2. Discussion of Background
As a flat panel display element (FPD) for coming generation, an organic EL display element has come into use for a cellular phone or the like. The organic EL display element comprises an organic luminescent material wherein a display takes place by its self-luminescence. Accordingly, it is more advantageous than conventional LCD or PDP in terms of quick response, visibility, luminance and so on.
The basic structure and the principle of operation are described in, for instance, “Appl. Phys. Lett., 51, 913 (1987). In order to cause luminescence, it has opposing electrodes at least one of which is a transparent electrode (made of, for instance, a tin-doped indium oxide (ITO)) and organic layers such as a hole transport layer, a light emission layer, an electron transport layer and so on are provided in this order from an anode side, between the opposing electrodes. Further research has been carried out so as to prolong the lifetime of the organic EL display element, to achieve an increased luminance, a full-colored display and so on.
The organic EL display element belongs to a current-driving type display. Particularly, in a passive-driving type organic EL display element, a current is supplied to it in only a selection period for each row, whereby the light emission layer emits light in response to this so that a display takes place. As a result, a large current flows into the electrodes unlike a case of using a voltage-driving type LCD.
For example, it is assumed a case that a panel having a pixel size of 300 μm×300 μm and 100 anodes is driven at a duty ratio of 1/64. The total amount of the current flowing into cathodes in a selection period is 172.8 mA in order to operate it at a luminous efficiency of 1 cd/A and an average luminance of 300 cd/m2.
With demands of a full-colored display and a high definition display in the flat panel display (FPD) in recent years, it is desired for the transparent electrode to have a further low resistance. However, reduction in the resistance of ITO used conventionally for LCD or the like approaches the limit. Accordingly, a low resistance wiring technique combining a metal having a low resistance with ITO, used widely in TFT-LCD and so on, has been introduced.
Thus, it is necessary to use the low resistance wiring technique for controlling voltage increase due to a large current between the cathode and the connection terminal in the organic EL display element. Generally, a structure that supplementary wires are provided between cathodes and connection terminals so that a current flows to the connection terminals through the supplementary wires, is adopted.
However, there are very strong demands to increase the size, precision and luminance of the display panel. In order to achieve these demands, it is necessary to reduce further resistances of the supplementary wires. As a low resistance wiring material for FPD, Al or an Al alloy is generally used. However, hillocks are apt to occur in using Al or the Al alloy, and an Al oxide is easily produced on its surface. Further, even when such material is electrically connected to another metal, the contact resistance is high and accordingly, it is difficult to use such material as it is.
Therefore, a technique that Al or an Al alloy is capped with Mo or a Mo alloy (an alloy of Mo and Cr, Ti, Ta, Zr, Hf or V) is often adopted (see for example, JP-A-13-311954 as a prior art document 1) because Mo can be etched with the same etching liquid as for Al. Accordingly, when a combination of Mo and Al is used, Al and Mo can be patterned together in a photolithographic process to form a display panel.
However, since the humidity resistance of Mo is generally low and it is easily corroded due to moisture in air, there was a problem that when Mo was used as a wiring material for FPD, the wire portions were apt to deteriorate. On the other hand, when Al was capped with Cr having a high humidity resistance, it could not be etched with the same etching liquid as for Al and therefore, there was difficulty in patterning both materials together in a manufacturing process.
Further, since Ni has a high humidity resistance, the resistance does not show a substantial change even when it is left under a high moisture condition. However, Ni can not actually be etched when a certain kind of etching liquid (comprising phosphoric acid:nitric acid:acetic acid and water=16:1:2:1 (in volume ratio)) is used. (“Photoetching and Microfabrication” by Kiyotake Naraoka and one other, published by Sogo Denshi Shuppansha (on May 10, 1977), p.82–p.83).
Further, since Ni is a ferromagnetic material, it is difficult to use a magnetron sputtering method as a generally used thin film forming method. Accordingly, it is difficult to use a thin Ni film as a wiring material for FPD.
Further, in the organic EL display element, the contact between a cathode and a supplementary wire and lowering of the resistance of a connection terminal to a supplementary wire create new problems. In particular, it is necessary for the contact characteristics between the cathode and the supplementary wire to have not only low resistance characteristics but also being stable against the Joule heat generated at the contact portion depending on the magnitude of an electric current flowing there.
Namely, the difficulty of increasing the contact resistance by the Joule heat is required. The increase of the contact resistance by the Joule heat is considered to be due to the oxidation of the metal used for the supplementary wire or the like.
FIG. 17 is a cross-sectional view partly omitted of an organic EL display element prepared according to a conventional technique. An anode 20a is formed on a transparent substrate 1 made of glass or the like. An electrode disposed inside the element and a driving circuit are connected with a supplementary wire 30×comprising a connection-terminal-side pattern portion 30b and an inner side pattern portion 30a. A cathode 70 is connected electrically to a connecting wire 150 at an outer side by means of the supplementary wire 30X. An organic EL layer 60 emits light by supplying a current between the anode 20a and the cathode 70. A counter substrate 80 is provided to seal the organic EL layer 60 and so on.
An insulation film 40 serves to define an opening region 40a where the organic EL layer 60 and the anode 20a contact. In such structure, generally, ITO (indium oxide-tin oxide) is used for the anode 20a, and an easily oxidizable metal such as Al, Mg, Ag or the like is used for the cathode. Metal such as Cr or the like is used for the supplementary wire.
When a patterned Cr having a film thickness of 300 nm, a width of 150 μm, a length of 4 mm and a specific resistance of 20 μΩ cm is used for the supplementary wire, the resistance is 17.7 Ω. In this case, when the above-mentioned current is supplied, a voltage drop of about 0.3.1 V takes place in response to the resistance of the wire, whereby there is a voltage increase beyond the predetermined electric potential.
Further, as shown in FIG. 17, an oxidized layer is formed on the surface of the supplementary wire 30×with processes of manufacturing, and accordingly, the contact resistance between the cathode 70 and the supplementary wire 30×increases to thereby increase a voltage in these members. The voltage rise is considered to cause adverse effects such as an uneven display at the time of gradation display and an increase in the breakdown of an anode driver used.
Explanation will be made as to a supplementary wiring technique described in JP-A-11-317292 (a prior art document 2). The prior art document 2 is characterized in that a transparent electrode material is used for a connection terminal connectable to a driving circuit, and the same material is used for a cathode and a supplementary wire. In this case, there would arise no problem about the contact resistance between the cathode and the supplementary wire unless surfaces of the cathode and the supplementary wire are oxidized before the connection of the cathode to the supplementary wire.
However, an easily oxidizable material is generally used for the cathode of an organic EL display element. Therefore, when the material for the supplementary wire is used also for the cathode, there occurs a problem that the surface of supplementary wire is oxidized during the manufacture of the organic EL display element and the contact resistance to the cathode becomes high. In particular, the increase of the contact resistance is remarkable when it is kept at a high temperature. When Al or an Al alloy is applied to the cathode or the supplementary wire, the contact resistance increases remarkably when it is kept at about 100° C.
JP-A-11-329750 (a prior art document 3) discloses a technique of reducing the contact resistance between a cathode and a supplementary wire. According to the proposal of the prior art document 3, low-resistance contact characteristics can be obtained by forming a supplementary wire portion into two portions: an undercoat pattern and an electrode pattern, and TiN or Cr is used for the undercoat pattern and Al is used for the electrode pattern to bring the supplementary wire portion into contact with the cathode.
In the prior art document 3, however, it is necessary to conduct a photolithographic process twice in order to form the supplementary wire portion. Further, in order to use TiN as a material for wiring, it is necessary to apply dry etching for the patterning which causes a problem of productivity. Further, in the case of using Cr for the undercoat pattern, the contact resistance may become extremely high when it is left at a high temperature of about 100° C. even though the initial contact characteristics are excellent.
In the organic EL display element, a large current should be supplied to the electrodes, and it is desirable to use a metal having low resistance characteristics is used to connect the cathode as described above. It is desirable that the connection terminal has excellent weather resistance characteristics, particularly, humidity resistance characteristics because it is not disposed in the sealed element but it is exposed in the environment.
Thus, when the material for the supplementary wire is used for the organic EL display element, there is a requirement of not only having excellent contact characteristics to the cathode but also being capable of suppressing corrosion due to moisture as possible because the material is extended from the sealed display panel to the outside of it.
The present invention is to provide a laminate applicable to an organic EL display element. It can form a substrate with wires having excellent humidity resistance, and it provides also a low resistance and an excellent patterning performance. Further, it is an object of the present invention to provide a substrate with wires formed by using the laminate.
In particular, the present invention is to provide a method for producing a substrate with wires by forming a laminate suitable for FPD such as an organic EL display element and etching the laminate two dimensionally, and to provide a substrate with wire formed by using the method.
Further, the present invention is to provide a circuit structure exhibiting excellent low resistance characteristics at a contact region constituting the circuit when a driving current is supplied to an organic EL display element to cause light emission.
Further, the present-invention is to maintain low resistance contact characteristics to the electrodes to which a large current is supplied and to realize reliable contact characteristics. Further, the present invention is to provide an organic EL display element of high reliability wherein the corrosion resistance of a metal constituting the electrodes and wires is improved.